Compressive properties of unidirectional carbon fibre laminates: II. The effects of unintentional and intentional fibre misalignments

Abstract

Unidirectional laminates were made with AS4 carbon reinforced PEEK from four different types of prepreg, one from ICI (APC 2) and three from BASF. They were sectioned at a small angle to the main fibre direction for the quantification of the adventitious misalignments produced by the prepregging and moulding operations. They were also compressive tested by using the compact flexure beam. The results showed that carbon reinforced PEEK composites are stronger than expected, and that these strengths and compressive moduli correlate with the degree of fibre misalignment. The greater the misalignment, the weaker and more compliant the laminate. The compressive moduli decreased with increasing strain while the tensile moduli increased. Laminates were also made with prepreg which had been crimped to various degrees to vary the fibre waviness in the composite. Wavelengths and amplitudes of waviness were estimated by using a microscope, and correlated with compressive strength and modulus. It was observed that fibre waviness decreased the compressive modulus approximately as the square of the mean fibre angular deviation. Compressive strength also decreased. The decrease in modulus with increasing strain appeared to be associated with the fibres themselves, rather than being caused by the fibre waviness.